@jdpnielsen/contextual-error

Contextual Error: Rich errors in Typescript with causes

The Contextual Error module is a rewrite of Joyent's excellent verror in Typescript. The port is not exactly a drop in replacement, as several features has been dropped, while others have been added.

This module extends the base Error class with several features which makes the Error class better suited for operational errors in modern Javascript workflows using promises in a async/await workflow. Please see Joyent's Best Practices for Error Handling in Node.js if you find any of the behavior here confusing or surprising.

The error classes here support:

• Chains of causes
• Properties to provide extra information about the error
• Creating your own subclasses that support all of these

The classes here are:

• CError, for chaining errors while preserving each one's error message. This is useful in servers and command-line utilities when you want to propagate an error up a call stack, but allow various levels to add their own context. See examples below.
• WError, for wrapping errors while hiding the lower-level messages from the top-level error. This is useful for API endpoints where you don't want to expose internal error messages, but you still want to preserve the error chain for logging and debugging.

Quick start

First, install the package:

npm install @jdpnielsen/contextual-error

If nothing else, you can use CError as a drop-in replacement for the built-in JavaScript Error class:

import CError from '@jdpnielsen/contextual-error';
var err = new CError('missing file: "/etc/passwd"');
console.log(err.message);

This prints:

missing file: "/etc/passwd"

You can also pass a cause argument, which is any other Error object:

import CError from '@jdpnielsen/contextual-error';
import fs from 'fs/promises';
var filename = '/nonexistent';

try {
  const stats = await fs.stat(filename); 
} catch (err1) {
	var err2 = new CError(`stat "${filename}"`, err1);
	console.error(err2.message);
}

This prints out:

stat "/nonexistent": ENOENT, stat '/nonexistent'

which resembles how Unix programs typically report errors:

$ sort /nonexistent
sort: open failed: /nonexistent: No such file or directory

To match the Unixy feel, when you print out the error, just prepend the program's name to the CError's message.

You can get the next-level Error using CError.cause():

console.error(CError.cause(err2).message);

prints:

ENOENT, stat '/nonexistent'

Of course, you can chain these as many times as you want, and it works with any kind of Error:

const err1 = new Error('No such file or directory');
const err2 = new CError(`failed to stat ${filename}`, err1);
const err3 = new CError('request failed', err2);
console.error(err3.message);

This prints:

request failed: failed to stat "/junk": No such file or directory

The idea is that each layer in the stack annotates the error with a description of what it was doing. The end result is a message that explains what happened at each level.

You can also decorate Error objects with additional information so that callers can not only handle each kind of error differently, but also construct their own error messages (e.g., to localize them, format them, group them by type, and so on). See the example below.

Deeper dive

The two main goals for CError are:

• Make it easy to construct clear, complete error messages intended for people. Clear error messages greatly improve both user experience and debuggability, so we wanted to make it easy to build them. That's why the constructor takes printf-style arguments.
• Make it easy to construct objects with programmatically-accessible metadata (which we call informational properties). Instead of just saying "connection refused while connecting to 192.168.1.2:80", you can add properties like "ip": "192.168.1.2" and "tcpPort": 80. This can be used for feeding into monitoring systems, analyzing large numbers of Errors (as from a log file), or localizing error messages.

To really make this useful, it also needs to be easy to compose Errors: higher-level code should be able to augment the Errors reported by lower-level code to provide a more complete description of what happened. Instead of saying "connection refused", you can say "operation X failed: connection refused". That's why CError supports causes.

In order for all this to work, programmers need to know that it's generally safe to wrap lower-level Errors with higher-level ones. If you have existing code that handles Errors produced by a library, you should be able to wrap those Errors with a CError to add information without breaking the error handling code. There are two obvious ways that this could break such consumers:

• The error's name might change. People typically use name to determine what kind of Error they've got. To ensure compatibility, you can create CErrors with custom names, but this approach isn't great because it prevents you from representing complex failures. For this reason, CError provides findCauseByName, which essentially asks: does this Error or any of its causes have this specific type? If error handling code uses findCauseByName, then subsystems can construct very specific causal chains for debuggability and still let people handle simple cases easily. There's an example below.
• The error's properties might change. People often hang additional properties off of Error objects. If we wrap an existing Error in a new Error, those properties would be lost unless we copied them. But there are a variety of both standard and non-standard Error properties that should not be copied in this way: most obviously name, message, and stack, but also fileName, lineNumber, and a few others. Plus, it's useful for some Error subclasses to have their own private properties -- and there'd be no way to know whether these should be copied. For these reasons, CError first-classes these information properties. You have to provide them in the constructor, you can only fetch them with the info() function, and CError takes care of making sure properties from causes wind up in the info() output.

Let's put this all together with an example from the node-fast RPC library. node-fast implements a simple RPC protocol for Node programs. There's a server and client interface, and clients make RPC requests to servers. Let's say the server fails with an UnauthorizedError with message "user 'bob' is not authorized". The client wraps all server errors with a FastServerError. The client also wraps all request errors with a FastRequestError that includes the name of the RPC call being made. The result of this failed RPC might look like this:

name: FastRequestError
message: "request failed: server error: user 'bob' is not authorized"
rpcMsgid: <unique identifier for this request>
rpcMethod: GetObject
cause:
    name: FastServerError
    message: "server error: user 'bob' is not authorized"
    cause:
        name: UnauthorizedError
        message: "user 'bob' is not authorized"
        rpcUser: "bob"

When the caller uses CError.info(), the information properties are collapsed so that it looks like this:

message: "request failed: server error: user 'bob' is not authorized"
rpcMsgid: <unique identifier for this request>
rpcMethod: GetObject
rpcUser: "bob"

Taking this apart:

• The error's message is a complete description of the problem. The caller can report this directly to its caller, which can potentially make its way back to an end user (if appropriate). It can also be logged.
• The caller can tell that the request failed on the server, rather than as a result of a client problem (e.g., failure to serialize the request), a transport problem (e.g., failure to connect to the server), or something else (e.g., a timeout). They do this using findCauseByName('FastServerError') rather than checking the name field directly.
• If the caller logs this error, the logs can be analyzed to aggregate errors by cause, by RPC method name, by user, or whatever. Or the error can be correlated with other events for the same rpcMsgid.
• It wasn't very hard for any part of the code to contribute to this Error. Each part of the stack has just a few lines to provide exactly what it knows, with very little boilerplate.

It's not expected that you'd use these complex forms all the time. Despite supporting the complex case above, you can still just do:

new CError("my service isn't working");

for the simple cases.

Reference: CError, WError

CError and WError are convenient drop-in replacements for Error that support first-class causes, informational properties and other useful features.

Constructors

The CError constructor takes 3 optional arguments:

new CError(message, cause, options)

All of these forms construct a new CError that behaves just like the built-in JavaScript Error class, with some additional methods described below.

In the first form, options is a plain object with any of the following optional properties:

Option name	Type	Meaning
name	string	Describes what kind of error this is. This is intended for programmatic use to distinguish between different kinds of errors. Note that in modern versions of Node.js, this name is ignored in the stack property value, but callers can still use the name property to get at it.
info	object	Specifies arbitrary informational properties that are available through the CError.info(err) static class method. See that method for details.

The WError constructor is used exactly the same way as the CError constructor.

Public properties

CError and WError all provide the same public properties as JavaScript's built-in Error objects.

Property name	Type	Meaning
name	string	Programmatically-usable name of the error.
message	string	Human-readable summary of the failure. Programmatically-accessible details are provided through CError.info(err) class method.
stack	string	Human-readable stack trace where the Error was constructed.

The stack property is managed entirely by the underlying JavaScript implementation. It's generally implemented using a getter function because constructing the human-readable stack trace is somewhat expensive.

Class methods

The following methods are defined on the CError class and as exported functions on the CError module. They're defined this way rather than using methods on CError instances so that they can be used on Errors not created with CError.

CError.isCError(err)

The isCError() function returns a boolean, if the provided error is an instance of CError or an instance of a class which inherited from CError. Under the hood, the method uses a Symbol for checking, rather than err instanceof CError. This allows compatability between versions of this library.

CError.cause(err)

The cause() function returns the next Error in the cause chain for err, or null if there is no next error. See the cause argument to the constructor. Errors can have arbitrarily long cause chains. You can walk the cause chain by invoking CError.cause(err) on each subsequent return value. If err is not a CError, the cause is null.

CError.info(err)

Returns an object with all of the extra error information that's been associated with this Error and all of its causes. These are the properties passed in using the info option to the constructor. Properties not specified in the constructor for this Error are implicitly inherited from this error's cause.

These properties are intended to provide programmatically-accessible metadata about the error. For an error that indicates a failure to resolve a DNS name, informational properties might include the DNS name to be resolved, or even the list of resolvers used to resolve it. The values of these properties should generally be plain objects (i.e., consisting only of null, undefined, numbers, booleans, strings, and objects and arrays containing only other plain objects).

CError.fullStack(err)

Returns a string containing the full stack trace, with all nested errors recursively reported as 'caused by:' + err.stack.

CError.findCauseByName(err, name)

The findCauseByName() function traverses the cause chain for err, looking for an error whose name property matches the passed in name value. If no match is found, null is returned.

If all you want is to know whether there's a cause (and you don't care what it is), you can use CError.hasCauseWithName(err, name).

If a vanilla error or a non-CError error is passed in, then there is no cause chain to traverse. In this scenario, the function will check the name property of only err.

CError.hasCauseWithName(err, name)

Returns true if and only if CError.findCauseByName(err, name) would return a non-null value. This essentially determines whether err has any cause in its cause chain that has name name.

Examples

The "Quick start" section above covers several basic cases. Here's a more advanced case:

const err1 = new CError('something bad happened');
/* ... */
const err2 = new CError( `failed to connect to "${ip}:${port}"`, err1, {
  'name': 'ConnectionError',
  'info': {
      'errno': 'ECONNREFUSED',
      'remote_ip': ip,
      'port': port
  }
});

console.log(err2.message);
console.log(err2.name);
console.log(CError.info(err2));
console.log(err2.stack);

This outputs:

failed to connect to "127.0.0.1:215": something bad happened
ConnectionError
{ errno: 'ECONNREFUSED', remote_ip: '127.0.0.1', port: 215 }
ConnectionError: failed to connect to "127.0.0.1:215": something bad happened
    at Object.<anonymous> (/home/jdpnielsen/cerror/examples/info.js:5:12)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:935:3

Information properties are inherited up the cause chain, with values at the top of the chain overriding same-named values lower in the chain. To continue that example:

const err3 = new CError('request failed', err2, {
  'name': 'RequestError',
  'info': {
      'errno': 'EBADREQUEST'
  }
});

console.log(err3.message);
console.log(err3.name);
console.log(CError.info(err3));
console.log(err3.stack);

This outputs:

request failed: failed to connect to "127.0.0.1:215": something bad happened
RequestError
{ errno: 'EBADREQUEST', remote_ip: '127.0.0.1', port: 215 }
RequestError: request failed: failed to connect to "127.0.0.1:215": something bad happened
    at Object.<anonymous> (/home/jdpnielsen/cerror/examples/info.js:20:12)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:935:3

You can also print the complete stack trace of combined Errors by using CError.fullStack(err).

var err1 = new CError('something bad happened');
/* ... */
var err2 = new CError('something really bad happened here', err1);

console.log(CError.fullStack(err2));

This outputs:

CError: something really bad happened here: something bad happened
    at Object.<anonymous> (/home/jdpnielsen/cerror/examples/fullStack.js:5:12)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:139:18)
    at node.js:968:3
caused by: CError: something bad happened
    at Object.<anonymous> (/home/jdpnielsen/cerror/examples/fullStack.js:3:12)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:139:18)
    at node.js:968:3

CError.fullStack is also safe to use on regular Errors, so feel free to use it whenever you need to extract the stack trace from an Error, regardless if it's a CError or not.